The expression of a dimerizable cre was achieved by insertion of the cre60.F2 N-terminal cre moiety which is fused to FRB (ligand binding domain of FRAP,an FKBP12-rapamycin associated protein) under the control of the endogenous promoter and a Caggs-cre59.F2 cassette, containing the C-teminal moiety of cre fused to FKBP12 (FK506-binding protein) under control of the CAG promoter, into the ROSA locus. A hygro cassette was also inserted into the locus for selection purposes. The split cre is inactive until dimerization is induced by administration of rapamycin. The hygro cassette was removed by crossing F1 Gt(ROSA)26Sortm1(cre)Jphe mice with FLPe mice.